1. Field of the Invention
The present invention relates to an alkaline storage battery such as a nickel-cadmium storage battery, a nickel-metal hydride storage battery or the like. More particularly, the present invention is concerned with an improvement of separators in an alkaline storage battery including a group of electrodes in which positive and negative electrode plates are spirally wound up with separators interposed therebetween.
2. Description of the Prior Art
Generally, in an alkaline storage battery such as a nickel-cadmium storage battery, a nickel-metal hydride storage battery or the like, positive and negative electrode plates are spirally wound up with separators interposed respectively therebetween to provide a group of electrodes, and a set of current collectors are connected to the opposite ends of the group of electrodes to provide a cylindrical electrode assembly. The electrode assembly is contained in a metallic cylindrical cell casing, and a lead plate extended from the current collector for the positive electrode is welded to a bottom of a closure element. Thereafter, the cell casing is supplied with an amount of electrolyte, and the closure element is coupled with an opening of the cell casing through an insulation gasket in a liquid-tight manner.
In a manufacturing process of a nickel-cadmium battery, for example, a nickel positive electrode plate is produced by impregnating an amount of nickel active material into a sintered nickel substrate plate in a chemical-permeation method, and a cadmium negative electrode plate is produced by impregnating an amount of cadmium active material into a sintered nickel substrate plate in the chemical-permeation method. Thereafter; the nickel positive electrode plate and cadmium negative electrode plate are spirally wound up with separators interposed therebetween to provide a group of spiral electrodes.
In recent years, there has been a demand for a higher capacity and a higher output m such an alkaline storage battery of this kind. To satisfy this demand, it is required to impregnate the active materials into the substrate plates at a higher density, and it is also required to form the separator as thin as possible. However, in a storage battery comprised of electrode plates impregnated with the active materials at a higher density and a separator formed as thin as possible, there is a problem that short-circuits will occur at a higher rate. To investigate the cause for such a short-circuit, a storage battery that suffered from a short-circuit was disassembled. As a result, it has been found that cracks and burrs in the positive electrode plate and damage of the active material in the positive electrode plate were caused at the outside of the positive electrode plate of the spiral electrode assembly, resulting in frequent occurrences of a short-circuit due to fragments or powders penetrating the separator. On the other hand, it has been confirmed that any short-circuit caused by cracks and burrs in the positive electrode plate and damage of the active material did not occur at all at the inside of the positive electrode plate of the spiral electrode assembly. In this respect, it is assumed that the positive electrode plate becomes fragile due to the active material impregnated therein at a higher density for increasing the capacity and output of the storage battery, resulting in the occurrence of cracks and burrs in the positive electrode plate and damage of the positive electrode plate in the winding process of the electrode plates, and it is also assumed that the strength of the separator is deteriorated due to a process for forming the separator as thin as possible, resulting in the occurrence of fragments or powders of the positive electrode plate penetrating therethrough.
It is, therefore, a primary object of the present invention to provide an alkaline storage battery wherein a separator placed at the outside of a positive electrode plate in a group of spiral electrodes is reinforced to eliminate an occurrence of short-circuit without causing any deterioration of the performance of the storage battery.
According to the present invention, the object is accomplished by providing an alkaline storage battery having a group of spiral electrodes composed of positive and negative electrode plates spirally wound with separators composed of a first separator placed at the outside of the positive electrode plate and a second separator placed at the inside of the positive electrode plate, and wherein the thickness of the first separator is made larger than that of the second separator.
In use of separators in the storage battery, the mechanical strength of the first separator placed at the outside of the positive electrode plate is reinforced to eliminate an occurrence of short-circuit in the storage battery caused by cracks and burrs in the positive electrode plate and damage of the active material without causing any deterioration of the battery performance such as characteristics in discharge capacity, operation voltage and the like.
In a practical embodiment of the present invention, it is preferable that the first separator is made of two layers, each thinner than the second separator but together formed into a composite first separator thicker than said second separator. In such an embodiment, the first and second separators are spirally wound with the positive electrode plate in such a manner that the two sheets of thin separators are placed on the outside of the positive electrode plate. Thus, the first separator can be provided thicker than the second separator in a simple manner during the winding process of the separators.
To reinforce the first separator, the basis weight of the first separator may be made larger than that of the second separator in such a manner that an occupancy ratio of the first separator becomes equal to that of the second separator. In this case, the mechanical strength of the first separator can be reinforced to prevent fragments or powders of the positive electrode plate from penetrating the separators thereby to eliminate an occurrence of short-circuit without causing any deterioration of the battery performance.
In a practical embodiment of the present invention, it is also preferable that the first and second separators are made of split short and long fibers of polyolefin resin uniformly entangled with each other, wherein the basis weight of the first separator is made larger than that of the second separator. In use of the separators, the surface area of the separators is increased by the split short fibers to enhance the retention property of electrolyte and to suppress an increase of the internal pressure of the storage battery, and the porosity of the separators is increased by the split long fibers to enhance the gas permeability of the separators. Accordingly, the separators are useful to eliminate an occurrence of short-circuit, to enhance the utilization ratio of the active material and to prevent an increase of the internal pressure of the storage battery.